400 - 0 - 400, with a cap input, is going to give you approx 560 volts before the losses of the 5U4. Spec sheet for the 5U4 says a loss of 50 volts/plate @ 275 ma /plate. Your draw is under 200 ma total, so ball park loss in the 5U4 maybe 50 volts? That would give you 560 - 50 volts = 510 to 500 volts. If I'm right, you'll still have a bunch of volts to drop.

A more general question:
Do you spec a transformer purely on the current drawn from the tubes, or are there other things to consider?
So if I want a winding that can power 2 300b's, can I just say '300b draws max 100mA so i need a 2x100mA = 200mA rating on that winding'?